The invention is directed to a chip assembly process, and more particularly to a process for connecting components to a substrate using solder balls and an underfill compound.
In a chip assembly process, electrical components are often attached to a substrate by connecting solder balls to a die, soldering the die to the substrate and then applying an underfill compound. The underfill compound is used to prevent damage to the assembled device caused by stresses from subsequent thermal variations.
In one known method, a die has a plurality of solder balls disposed on one surface for attachment to pads or traces on a substrate. Previously known methods involved applying a controlled amount of flux to a portion of the solder balls, soldering the die to the substrate via the solder balls, and then applying an underfill liquid at the die perimeter. The amount and location of the flux is carefully controlled because the amount and location of the flux determines the extent and volume of solder that will flow out from the solder balls onto the substrate in the solder joint. The underfill liquid is distributed through the solder joint by being drawn into the gap between the substrate and the die via capillary action and then cured in a time and temperature controlled process. The problem with this method is that the underfilling time and subsequent cure schedule are quite long compared to the other steps in the chip assembly process.
In an attempt to reduce the assembly cycle time, another known process shown in FIGS. 1 and 2 incorporates a fluxing underfill compound 108 in the chip assembly before the die and substrate are positioned together. No flux is applied to the solder balls 102 in this case because the underfill compound 108 is formulated to provide the fluxing action to allow assembly of the solder joint 110. Because the fluxing action is distributed throughout the entire volume of the underfill material 108, however, the solder balls 102 tend to collapse after the die 100 is attached to the substrate 106, as shown in FIG. 2. This is because the flux location is not selectively applied to the solder balls 102 and because the boundaries of the solder flow out area have not been defined by a resist system on the substrate 106. The collapsed solder balls 102 tend to cause the solder joint profile 110 to vary from chip to chip because there is no way to control the degree of wetting out of the solder joint 110. Further, the collapsed solder balls 102 may lead to early fatigue failures in the joint 110 during subsequent thermal cycling.
There is a need for a chip assembly method that reduces the time required for the underfill operation while maintaining a consistent, repeatable, and optimum solder joint profile.
Accordingly, the present invention is directed to a chip assembly method that includes the steps of applying flux to at least a portion of the solder balls on the die, applying a non-fluxing underfill to the substrate, and then attaching the solder balls to the substrate, preferably on pad or trace areas disposed on the substrate. By applying flux to the solder balls and depositing non-fluxing underfill material onto the substrate before the die and substrate are connected together rather than afterward, the inventive method increases the speed of the underfill stage in a chip assembly process while maintaining a consistent solder joint profile and sufficient clearance between the die and substrate, thereby minimizing early fatigue failures in the joint.